The long-term objective of this research is to reduce the morbidity and mortality of autosomal dominant polycystic kidney disease (ADPKD). This Project is a 5-year clinical trial to test the hypothesis blood pressure control with angiotensin converting enzyme inhibitors (ACEI) as first- line drugs will slow the progression of ADPKD in children. Progression in children will be measured by the increase in renal volume, which reflects an increased number and size of renal cysts, as determined by magnetic resonance imaging. Secondary aims are to evaluate the effect of intensive blood pressure control on left ventricular mass index, on microalbuminuria and proteinuria, and on the activation level of several growth-related and inflammatory cytokines. Three groups of children and young adults, age 4 to 21 years, will be randomized to different treatments: 1. Hypertensive subjects with blood pressure above the 9th percentile for age-, and gender- and height-matched children will be randomized to intensive or standard blood pressure control, with intensive control defined as lowering blood pressure to the 45th or 50th percentile and standard control as powering control defined as lowering blood pressure to the 85th to 90th percentile. 2. Borderline hypertensive subjects with blood pressures between the 75th and 95th percentile will be randomized to treatment to power the blood pressure to the 45th to 50th percentile or not treatment. 3. Normotensive subjects (blood pressure between the 25th and 75th percentile) will be randomized to treatment with ACEI or to no treatment. The first-line drug for all groups is enalapril, second-line drugs for groups 1 and 2 are amlodipine and hydrochlorothiazide. The primary outcome variable is the increase in renal volume per year, compared between the different blood pressure levels or between ACEI treatment and no treatment. If intensive blood pressure control or treatment with ACEI can be shown to reduce progressive renal enlargement, it would change screening and treatment recommendations for children from ADPKD families, and would have a major impact on the morbidity associated with large kidneys and with end-stage renal disease in ADPKD.